全文获取类型
收费全文 | 1288篇 |
免费 | 76篇 |
出版年
2023年 | 6篇 |
2022年 | 4篇 |
2021年 | 17篇 |
2020年 | 23篇 |
2019年 | 21篇 |
2018年 | 38篇 |
2017年 | 36篇 |
2016年 | 44篇 |
2015年 | 54篇 |
2014年 | 61篇 |
2013年 | 92篇 |
2012年 | 103篇 |
2011年 | 87篇 |
2010年 | 89篇 |
2009年 | 58篇 |
2008年 | 67篇 |
2007年 | 59篇 |
2006年 | 61篇 |
2005年 | 80篇 |
2004年 | 64篇 |
2003年 | 57篇 |
2002年 | 42篇 |
2001年 | 10篇 |
2000年 | 13篇 |
1999年 | 7篇 |
1998年 | 12篇 |
1997年 | 16篇 |
1996年 | 9篇 |
1995年 | 8篇 |
1994年 | 8篇 |
1993年 | 12篇 |
1992年 | 5篇 |
1991年 | 8篇 |
1990年 | 4篇 |
1989年 | 8篇 |
1988年 | 5篇 |
1987年 | 3篇 |
1985年 | 6篇 |
1984年 | 7篇 |
1983年 | 4篇 |
1982年 | 5篇 |
1981年 | 10篇 |
1980年 | 4篇 |
1978年 | 3篇 |
1977年 | 6篇 |
1966年 | 3篇 |
1962年 | 2篇 |
1961年 | 2篇 |
1960年 | 4篇 |
1958年 | 2篇 |
排序方式: 共有1364条查询结果,搜索用时 15 毫秒
991.
Protein kinase D (PKD) plays an important role in mediating cellular DNA synthesis in response to G protein-coupled receptor (GPCR) agonists but the function of other isoforms of the PKD family has been much less explored. Here, we examined whether PKD2 overexpression in Swiss 3T3 cells facilitates DNA synthesis and the activation of the extracellular regulated protein kinase (ERK) pathway in response to the mitogenic GPCR agonist bombesin. We show that PKD2 overexpression markedly potentiated the ability of this agonist to induce DNA synthesis. Addition of bombesin to Swiss 3T3 cells overexpressing PKD2 also induced a striking increase in the duration of MEK/ERK/RSK activation as compared with cultures of control cells. In contrast, neither DNA synthesis nor the duration of ERK activation in response to epidermal growth factor, which acts via protein kinase C/PKD2-independent pathways, was increased. Furthermore, bombesin promoted a striking accumulation of c-Fos protein in cells overexpressing PKD2. Our study demonstrates that PKD2, like PKD, facilitates mitogenesis and supports the hypothesis that an increase in the duration of the ERK signaling leading to accumulation of immediate gene products is one of the mechanisms by which isoforms of the PKD family enhance re-initiation of DNA synthesis by Gq-coupled receptor activation. 相似文献
992.
Okuda K de Almeida F Mortara RA Krieger H Marinotti O Bijovsky AT 《Journal of insect physiology》2007,53(12):1307-1315
Haematophagy, the utilization of blood as food, has evolved independently among insects such as mosquitoes, bedbugs, fleas, and others. Accordingly, several distinct biological adaptations have occurred in order to facilitate the finding, ingestion and digestion of blood from vertebrate sources. Although blood meals are essential for survival and reproduction of these insects, mechanical and chemical stresses are caused by the ingestion of a sizable meal (frequently twice or more times the weight of the insect) containing large amounts of cytotoxic molecules such as haem. Here we present data showing that the stresses caused by a blood meal induce cell death in the midgut epithelium of Culex quinquefasciatus mosquitoes. The process involves apoptosis, ejection of dead cells to the midgut lumen and differentiation of basal regenerative cells to replace the lost digestive cells. The basal cell differentiation in blood-fed mosquito midguts represents an additional mechanism by which insects cope with the stresses caused by blood meals. C. quinquefasciatus adult females are unable to replace lost cells following a third or fourth blood meal, which may have a significant impact on mosquito longevity, reproduction and vectorial capacity. 相似文献
993.
The early protective thymus-independent antibody response to foot-and-mouth disease virus is mediated by splenic CD9+ B lymphocytes 下载免费PDF全文
Ostrowski M Vermeulen M Zabal O Zamorano PI Sadir AM Geffner JR Lopez OJ 《Journal of virology》2007,81(17):9357-9367
Infection of mice with cytopathic foot-and-mouth disease virus (FMDV) induces a rapid and specific thymus-independent (TI) neutralizing antibody response that promptly clears the virus. Herein, it is shown that FMDV-infected dendritic cells (DCs) directly stimulate splenic innate-like CD9(+) B lymphocytes to rapidly (3 days) produce neutralizing anti-FMDV immunoglobulin M antibodies without T-lymphocyte collaboration. In contrast, neither follicular (CD9(-)) B lymphocytes from the spleen nor B lymphocytes from lymph nodes efficiently respond to stimulation with FMDV-infected DCs. The production of these protective neutralizing antibodies is dependent on DC-derived interleukin-6 (IL-6) and on CD9(+) cell-derived IL-10 secretion. In comparison, DCs loaded with UV-inactivated FMDV are significantly less efficient in directly stimulating B lymphocytes to secrete TI antibodies. A critical role of the spleen in the early production of anti-FMDV antibodies in infected mice was also demonstrated in vivo. Indeed, either splenectomy or functional disruption of the marginal zone of the spleen delays and reduces the magnitude of the TI anti-FMDV antibody response in infected mice. Together, these results indicate that in addition to virus localization, the FMDV-mediated modulation of DC functionality is a key parameter that collaborates in the induction of a rapid and protective TI antibody response against this virus. 相似文献
994.
995.
996.
Pérez-Pereira Noelia Wang Jinliang Quesada Humberto Caballero Armando 《Biodiversity and Conservation》2022,31(11):2763-2780
Biodiversity and Conservation - The establishment of the minimum size for a viable population (MVP) has been used as a guidance in conservation practice to determine the extinction risks of... 相似文献
997.
998.
High abundances of aerobic anoxygenic photosynthetic bacteria in the South Pacific Ocean 总被引:2,自引:0,他引:2
Lami R Cottrell MT Ras J Ulloa O Obernosterer I Claustre H Kirchman DL Lebaron P 《Applied and environmental microbiology》2007,73(13):4198-4205
Little is known about the abundance, distribution, and ecology of aerobic anoxygenic phototrophic (AAP) bacteria, particularly in oligotrophic environments, which represent 60% of the ocean. We investigated the abundance of AAP bacteria across the South Pacific Ocean, including the center of the gyre, the most oligotrophic water body of the world ocean. AAP bacteria, Prochlorococcus, and total prokaryotic abundances, as well as bacteriochlorophyll a (BChl a) and divinyl-chlorophyll a concentrations, were measured at several depths in the photic zone along a gradient of oligotrophic conditions. The abundances of AAP bacteria and Prochlorococcus were high, together accounting for up to 58% of the total prokaryotic community. The abundance of AAP bacteria alone was up to 1.94 x 10(5) cells ml(-1) and as high as 24% of the overall community. These measurements were consistent with the high BChl a concentrations (up to 3.32 x 10(-3) microg liter(-1)) found at all stations. However, the BChl a content per AAP bacterial cell was low, suggesting that AAP bacteria are mostly heterotrophic organisms. Interestingly, the biovolume and therefore biomass of AAP bacteria was on average twofold higher than that of other prokaryotic cells. This study demonstrates that AAP bacteria can be abundant in various oligotrophic conditions, including the most oligotrophic regime of the world ocean, and can account for a large part of the bacterioplanktonic carbon stock. 相似文献
999.
Ghazi I Fernandez-Arrojo L Garcia-Arellano H Ferrer M Ballesteros A Plou FJ 《Journal of biotechnology》2007,128(1):204-211
A fructosyltransferase present in Pectinex Ultra SP-L, a commercial enzyme preparation from Aspergillus aculeatus, was purified to 107-fold and further characterised. The enzyme was a dimeric glycoprotein (20% (w/w) carbohydrate content) with a molecular mass of around 135 kDa for the dimer. Optimal activity/stability was found in the pH range 5.0-7.0 and at 60 degrees C. It was stable or slightly activated (upto 1.4-fold) in the presence of reducing agents, such as dithiothreitol and 2-mercaptoethanol, and detergents, such as sodium dodecylsulphate and Tween 80. The enzyme was able to transfer fructosyl groups from sucrose as donor producing the corresponding series of fructooligosaccharides: 1-kestose, nystose and fructosylnystose. Using sucrose as substrate, the k(cat) and K(m) values for transfructosylating activity were 1.62+/-0.09 x 10(4)s(-1) and 0.53+/-0.05 M, whereas for hydrolytic activity the corresponding values were 775+/-25s(-1) and 27+/-3 mM. At elevated sucrose concentrations, the fructosyltransferase from A. aculeatus showed a high transferase/hydrolase ratio that confers it a great potential for the industrial production of prebiotic fructooligosaccharides. 相似文献
1000.
Adsorption of heavy metal ion from aqueous single metal solution by chemically modified sugarcane bagasse 总被引:6,自引:0,他引:6
Karnitz O Gurgel LV de Melo JC Botaro VR Melo TM de Freitas Gil RP Gil LF 《Bioresource technology》2007,98(6):1291-1297
This work describes the preparation of new chelating materials derived from sugarcane bagasse for adsorption of heavy metal ions in aqueous solution. The first part of this report deals with the chemical modification of sugarcane bagasse with succinic anhydride. The carboxylic acid functions introduced into the material were used to anchor polyamines, which resulted in two yet unpublished modified sugarcane bagasse materials. The obtained materials were characterized by elemental analysis and infrared spectroscopy (IR). The second part of this reports features the comparative evaluation of the adsorption capacity of the modified sugarcane bagasse materials for Cu(2+), Cd(2+), and Pb(2+) ions in aqueous single metal solution by classical titration. Adsorption isotherms were studied by the Freundlich and Langmuir models. 相似文献